Langendorff hanging heart preparations (isolated, perfused, functioning rat hearts) have been shown to suffer functional damage under conditions of ischemia. Various treatments have been imposed upon the hearts to minimize the functional damage during such ischemic periods. In order to assess the biochemical changes occurring during these ischemic periods, the heart tissue has been extracted and the extracts have been examined for enzymic activities and for carbonyl content. Conditions of ischemia which are serious enough to provide only a 50 percent recovery of function, cause little change in creatine kinase (CPK) or isocitrate dehydrogenase activities, whether during the period of ischemia or during subsequent reperfusion. At the same time, however, lactate dehydrogenase activities rise during the ischemia and return to initial values upon reperfusion and glutamate-oxalacetate transaminase activities rise somewhat and then drop during ischemia. Carbonyl content, a measure of oxidative damage, undergoes a modest increase during ischemia but rises dramatically upon reperfusion, consistent with the idea that the free radical production and associated oxidation occurs upon the reintroduction of oxygen to the ischemic tissue. During the period of ischemia, there is a marked loss of ATP accompanied by rises in AMP and its breakdown products, inosine, hypoxanthine, and xanthine. Upon reperfusion, the ATP levels rise to near initial values and the AMP, xanthine, and hypoxanthine levels return to their original low levels. Interestingly, ADP levels change very little during the periods of ischemia or reperfusion. There are no apparent ischemia-dependent changes in isozyme distribution for creatine kinase, but lactate dehydrogenase demonstrates a definite shift from the heart-associated, more aerobic monomers to the liver-associated, less aerobic monomers. Upon reperfusion, the distribution of the monomers returns to that seen prior to ischemia.